Thermal transfer printers are well known in the prior art. In such printers, a transfer ribbon typically having a polyester backing coated on one side with a heat-transferable ink layer is interposed between the surface of a nonsensitized paper and a thermal printhead having a line of very small heater elements. When an electrical pulse is applied to a selected subset of the heater elements, localized melting and transfer of the ink to the paper occurs under the energized elements, resulting in a corresponding line of dots being transferred to the paper.
After each line of dots is printed, the paper or the printhead is repositioned to locate the printhead over an adjacent location and the transfer ribbon is repositioned to provide a replenished ink coating. Then the selecting and heating process is repeated to print an adjacent line of dots. Depending on the number and pattern of heater elements and the directions of motion of the thermal head and the paper, arrays of dots can produce individual characters or successive rows of dots which combine to form complete printed lines of text or graphics. It is common to transfer essentially all of the ink from the transfer ribbon to the paper and reposition the transfer ribbon by at least the dimension of the dots printed.
An alternate mode of operation is to increase the ink coating thickness and limit the heat energy so as to transfer only a portion of the ink layer to the paper at each dot line. Various attempts have thus been made to minimize consumption of transfer ribbon. One attempt includes the use of a "multipass" transfer ribbon with a thermal printer that has a removable, reversible ribbon carriage. Such ribbon carriage permits the user to remove the transfer ribbon at the end of each pass and reverse it, thereby permitting reuse of the transfer ribbon. Successful reuse has been reported up to five passes when used for printing bar codes and ten passes when used for printing fonts only. In an alternative arrangement, the thermal printer is adapted to rewind the partially used transfer ribbon automatically at the end of each pass. These approaches result in a complicated structure for the thermal printer because of the need to provide a removable ribbon spindle carriage to permit ribbon reversal or a special head lift mechanism to accommodate ribbon rewind. Moreover, the former arrangement requires user intervention to manually remove and reverse the ribbon carriage five to ten times over its lifetime.
Another approach has been to control the ribbon advance feed rate relative to the paper advance feed rate to extend the life of the transfer ribbon. By using this mode, the transfer ribbon can be repositioned by less than the thickness of the dot line, resulting in a reduced consumption of transfer ribbon.
In U.S. Pat. No. 4,558,963 to S. L. Applegate et al., conservation of ribbon in an impact printer is achieved by underfeeding the ribbon relative to movement of the printhead to provide a low ratio of ribbon speed to paper speed for drafts and a high ratio for final versions. However, this patent does not disclose continuously variable ratios or automatic control of such ratios. In U.S. Pat. No. 3,984,809 to Dertouzos et al., there is disclosed a thermal-transfer printer in which the ribbon moves slower than the paper. A roller is driven and moves the ribbon in contact therewith. However, the ribbon contacts the driven roller only for about 90.degree. of its circumference. In Japanese patent No. 199972 there is disclosed an image former in which transfer ribbon is pressed against a platen roller by a pair of idler rollers. However, the extent of contact of the transfer ribbon with the platen roller is much less than 180.degree. and the platen roller is not driven.
Effective use of the transfer ribbon is directly related to the rate of advancement of the transfer ribbon during a printing operation. If the transfer ribbon is advanced too slowly the increased time during which a given area of transfer ribbon is heated combined with the back tension from the ribbon take-up spool will result in stretching of the transfer ribbon at the point of heating. If the image being printed requires heating across much of the transfer ribbon width, the ribbon will break. This problem is negligible for small text because only a small number of heater elements are energized on each dot line. Printing of bar codes, large block letters, or graphics (hereinafter sometimes collectively referred to has "graphic-type images"), however, requires energizing adjacent blocks of heater elements or a significant number of the total heater elements or a significant number of the total heater elements for a number of several consecutive dot rows. Generally this will result in the breaking of the transfer ribbon unless the ribbon feed rate is increased when such images are printed. If the ribbon is advanced too fast, some inked portions will not be used, thereby wasting portions of the ribbon.